starfleet had already made the transition to the new uniforms when Voyager put to space.... either that or Bob Blackman decided on a larger wardrobe....

Actually, here's what happened. New uniforms were made for Generations, but ended up ultimately not being used, as the producers didn't want too many changes from TNG (although they instead had the crew change from the TNG uniforms to the DS9 uniforms halfway through the movie, which was even more awkward than having new uniforms right from the start )

Playmates actually released an action figure line with the TNG crew wearing the new uniforms, based on concept art: